The prior art shows conductive probes for measuring the level of conductive materials within conductive vessels. In general, the prior art shows probes having conductive cores and insulative coatings which are hung vertically within containers typically containing conductive liquids. The insulative coating acts as a dielectric material between the conductive core and the conductive liquid, thus allowing that portion of the insulation which is in the conductive liquid to act as the dielectric of a capacitor. The capacitance between the conductive core of the probe and the conductive vessel within which the conductive liquid is contained is therefore dependent on the depth to which the probe is immersed in the conductive liquid and can be measured, providing a signal proportional to the depth to which the probe is immersed in the conductive liquid. This approach is found in, inter alia, U.S. Pat. Nos. 3,706,980 to Maltby and 4,064,753 to Maltby and Sun, both of which are assigned to the assignee of the present application.
What is not found in the prior art is a method for measuring the level of a conductive granular material within a vessel. It will be appreciated by those skilled in the art that a conductive liquid within which a probe is immersed will tend to form a equipotential surface around the insulative surface of the probe to the depth to which the probe is immersed in the liquid. However, a conductive granular material such as coal, which may be in chunks as large as several inches across, will only contact the insulative coating of the probe at certain points, thus not providing an equipotential surface from which an accurate capacitance may be derived, thus not providing an accurate measurement. Therefore, there has, prior to the present invention, been no accurate means of measuring the level of a conductive granular material within a conductive vessel by means of this capacitance-probe technique. The present invention is designed to solve this problem. In addition, the present invention can be used in the conventional way for measurement of the level of conductive liquids or other nongranular conductive materials. The probe according to the present invention can also be adapted to provide a signal that a conductive granular material is or is not present at the probe rather than a varying signal dependent on the height of the conductive granular material along the probe; or a series of such one-level probes can then be used to determine the level of a granular material within a vessel.